Mycobacterium tuberculosis (M. tuberculosis) is a bacterium that causes tuberculosis (TB) in humans. TB is a disease that primarily affects the lungs, although it can attack other parts of the body. It spreads much like a cold or the flu — through the expelled airborne droplets of a person with infectious TB.

When inhaled, the bacterium can settle in the lungs, where it begins to grow. If not treated, it can spread to areas such as the kidneys, spine, and brain. It can be life-threatening.

According to the Centers for Disease Control and Prevention (CDC), nearly 10,000 new cases of TB were reported in the United States in 2016.

Millions of people harbor M. tuberculosis. According to the University of Iowa Environmental Health and Safety, one-third of the world’s population carries the bacterium, but not all of them get sick.

In fact, says the university, only 5 to 10 percent of those carrying the bacterium will actually develop a case of active, contagious tuberculosis in their lifetimes. That normally happens when the lungs are already damaged from diseases like chronic obstructive pulmonary disease (COPD) and cystic fibrosis or from smoking.

People also develop TB more easily when their immune system is weakened. Those undergoing chemotherapy for cancer, for example, or those who have HIV, may have weaker immune systems. The CDC reports that TB is a leading cause of death for people with HIV.

While both M. tuberculosis and Mycobacterium avium complex can cause lung disease, often with similar symptoms, they’re not the same.

M. tuberculosis causes TB. MAC causes lung diseases, such as a chronic infection of the lungs, but it doesn’t cause TB. It’s part of a group of bacteria known as NTM (nontuberculous mycobacteria).

M. tuberculosis is spread through the air. MAC is a common bacterium found primarily in water and soil. You can contract it when you drink or wash with contaminated water or handle soil or eat food with MAC-containing particles on it.

You can get M. tuberculosis when you breathe in expelled droplets from a person with an active TB infection. Symptoms of the disease include:

  • a bad, lingering cough
  • coughing up blood
  • pain in the chest
  • fever
  • fatigue
  • night sweats
  • weight loss

A person can have the bacterium but not have any symptoms. In this case, they aren’t contagious.

According to a recent study, 98 percent of cases are transmitted from the cough of a person with an active infection. These droplets can also become airborne when a person sneezes or speaks.

TB, however, is not simple to catch. According to the CDC, you can’t get it from a handshake, drinking from the same glass, or passing by a person with TB who is coughing. Rather, the bacterium is spread with more prolonged contact. For example, sharing a home or a long car ride with someone with an active infection may lead to you catching it.

While tuberculosis is on the downswing in the United States, it’s far from wiped out. Having a weakened immune system or lungs is a risk factor for developing TB.

It’s also a risk factor to have been recently exposed to TB. The CDC reports that 14 percent of TB cases in the United States are due to a recent transmission.

According to the CDC, those most likely to have been recently exposed include:

  • a close contact of someone with infectious TB
  • a person working or living with people who are themselves at high risk of TB infection (which includes people who work in hospitals, homeless shelters, or correctional facilities)
  • a person who’s immigrated from a part of the world with high levels of TB infection
  • a child under the age of 5 with a positive TB test

If you have symptoms of TB or you have risk factors, your doctor may order tests that look for exposure to M. tuberculosis. These tests can include:

  • Mantoux tuberculin skin test (TST). A fluid called tuberculin is injected under the skin of the arm. If you’ve been infected with M. tuberculosis, a reaction will occur within 72 hours of having the test.
  • Blood test. This measures your immune reaction to M. tuberculosis.

These tests only show whether or not you’ve been exposed to the TB bacterium, not whether you have an active case of TB. To determine that your doctor may order:

  • Chest X-ray. This allows the doctor to look for the kinds of lung changes TB produces.
  • Sputum culture. Sputum is a mucus and saliva specimen coughed up from your lungs.

People — even those in good health — cough and sneeze. To reduce your risk of acquiring M. tuberculosis as well as a host of other viruses and bacteria, follow these guidelines:

  • Take care of your health. According to the group TB Alert, 60 percent of people with a healthy immune system can fend off a TB infection. Eat a nutritious, well-balanced diet. Sleep seven to eight hours a night. Get regular exercise.
  • Keep your home and office well ventilated. That can help scatter any infected, expelled droplets.
  • Sneeze or cough into a tissue. Instruct others to do so as well.
  • Ask your doctor about getting the TB vaccine. While it’s not widely used in the United States, TB Alert reports that the TB vaccine is 80 percent effective in preventing TB for 15 years. Talk to your doctor about the pros and cons of receiving it.

According to the CDC, TB killed one out of every seven people in the United States and Europe in the early 1900s. Luckily, that’s changed. Nowadays, infection with M. tuberculosis is rare in healthy people in the United States. It does pose a serious risk for those who have compromised immune systems and lungs weakened by disease or environmental damage. (Health care workers are also at a higher risk.)

The bacterium is generally transmitted person to person via the inhalation of infected droplets. It’s also possible to get an infection when the bacterium passes through breaks in the skin or the mucus membranes.

The disease that M. tuberculosis produces can be deadly. But today, good medication — including the antibiotics isoniazid and rifampin — provide effective treatment.